There are about 5000 species of marine isopods, but less than 20 free-living ones are common in intertidal and supratidal regions of the west coast of North America. Owing to their generally nocturnal habits and preference for under-rock and under-seaweed habitats, many species are not obvious to the casual observer. An equal or greater number of species are parasitic or wood-boring (with a few exceptions the parasitic species are not included in the ODYSSEY). Isopods are of special interest because terrestrial representatives frequent our gardens (woodlice and sowbugs), and transitional evolutionary forms exist from the more advanced terrestrial species to prototypal land colonisers living in the supratidal region of west-coast shores.